Magnetic recording medium underlayer and method for obtaining a desired lattice parameter therefor

ABSTRACT

An underlayer ( 8 A) of a magnetic recording medium ( 16 ) includes first and second non-magnetic, chromium-based layers ( 18, 20 ), at least one of the first and second chromium-based layers being a chromium alloy. The lattice parameter of the composite underlayer is between the lattice parameters of the first and second chromium-based layers. Recognizing this permits one to predictably adjust the lattice parameter of the composite underlayer to be close to the lattice parameter of the magnetic layer ( 10 ) so to optimize magnetic and parametric properties.

BACKGROUND OF THE INVENTION

[0001] Magnetic thin film recording disks are commonly manufactured by a series of sputtering processes in an environment of low pressure inert gases. Disk 2, see FIG. 1, is commonly comprised of a substrate 4 made of nickel phosphorus (NiP) or ceramic glass substrate, an optional magnetic seed layer 6, a non-magnetic underlayer 8, made of either pure chromium (Cr) or a chrome alloy (CrX), covered by a magnetic layer 10 made of a cobalt (Co)-based alloy. A protective layer 12 made of sputtered carbon is typically used on top of the magnetic layer and an organic lubricant 14 may be used on top of the protective layer. It has been found that it is important to match the underlayer and magnetic layer lattice parameters for thin films in longitudinal recording. In the case of body center cubic (bcc) Cr or CrX underlayer, the lattice dimension is defined by the parameter aCr. For hexagonal close-packed (hcp) Co alloy, the lattice dimension is defined by two orthogonal vectors: a Co and cCo. See FIGS. 1A-1D. For a well-matched Co/Cr film stack, {square root}2 a c_(r)=c_(Co).

[0002] Lattice matching between the Cr or CrX underlayer and the Co alloy magnetic layer is commonly done to achieve optimal magnetic and parametric properties. See D. E. Laughlin et al., Scripta Metallurgica et Materialia 33, 1525 (1995).

[0003] Common Co alloys include chromium (Cr), platinum (Pt), boron (B), niobium (Nb), tungsten (W) and tantalum (Ta). While the cobalt platinum alloy has a hexagonal close packed (hcp) crystalline structure, the chromium-based underlayer has a body centered cubic (bcc) lattice structure. With the addition of platinum, the CoPt-based alloys have a larger lattice structure than the chromium-based underlayer so that elements such as Vanadium (V), Molybdenum (Mo), titanium (Ti) and tungsten (W) have been used to create chromium-based alloys to accommodate the larger lattice of the CoPt-based magnetic layers and maintain lattice union across the interface between the two. These alloys expand the unit cell structure so as to improve the lattice matching between the underlayer and the magnetic film because of the larger sized atoms of the alloying elements.

[0004] One of the problems with the conventional method for trying to match the lattice perimeter of the chromium-based underlayer with the CoPt-based magnetic layers is that it requires incremental experimentation with chromium alloying as to both what is used and the percentages. Therefore, each time the composition of the cobalt-based magnetic layer changes, which occurs relatively often as improved magnetic layers are developed, the composition of the chromium-based underlayer must be changed to complement the changed composition of the magnetic layer. However, this composition adjustment is tedious, time-consuming and, thus, expensive.

SUMMARY OF THE INVENTION

[0005] The present invention is directed to an underlayer of a magnetic recording medium in which the underlayer includes first and second non-magnetic, chromium-based layers, at least one of the first and second chromium-based layers being a chromium alloy. The lattice parameter of the composite underlayer is between the lattice parameters of the first and second chromium-based layers. Recognizing this permits one to predictably adjust the lattice parameter of the composite underlayer to be close to the lattice parameter of the magnetic layer so to optimize magnetic and parametric properties.

[0006] Another aspect of the invention relates to a method for obtaining a desired lattice parameter of a non-magnetic underlayer in the manufacture of a magnetic recording medium of the type comprising a substrate, a non-magnetic underlayer on the substrate, and a magnetic layer on the underlayer. The method includes determining a desired lattice parameter, typically the lattice parameter of the magnetic layer, for the underlayer. A number of non-magnetic, chromium-based layers, typically two, are selected for the underlayer, at least one of the chromium-based layers is a chromium alloy. The thickness of each chromium-based layer is chosen according to the desired lattice parameter, the sum of the thicknesses of each of the chromium-based layers being equal to the underlayer thickness. One way of choosing the thicknesses of the chromium-based layers is to use a plot of the ratio of the two thicknesses of the two chromium-based layers versus corresponding (002) interplanar spacing as the lattice parameter. The choosing step is preferably carried out so that the difference between the lattice parameter of the underlayer and the desired lattice parameter, typically the lattice parameter of the magnetic layer, is minimized. The final lattice parameter of this composite underlayer lies between the lattice parameters of the two respective Cr or CrX layers.

[0007] In some embodiments a protective layer is deposited on the magnetic layer, a lubricant is deposited on the protective layer and a non-magnetic seedlayer layer is provided between the substrate and the underlayer. The thickness of the underlayer is preferably less than about 350 Å, and more preferably between about 180-200 Å. The chromium-based layers are preferably between about 10-500 Å thick. If desired, more than two chromium-based layers may be used in making up the underlayer.

[0008] Other features and advantages of the invention will appear from the following description in which the preferred embodiment has been set forth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic representation of a conventional thin-film magnetic recording medium, typically a thin-film disk;

[0010] FIGS. 1A-1D illustrate epitaxy between Co and Cr with matching of interatomic distances;

[0011]FIG. 2 is a plot of calculated average atomic radius for CrX bcc layers versus lattice parameter, in particular x-ray diffraction (002) interplanar spacing;

[0012]FIG. 3 is a schematic illustration of a magnetic recording medium made according to the invention including first and second chromium-based layers constituting the underlayer;

[0013]FIG. 4 plots x-ray diffraction spectra of CrV20/Cr films for different film thickness ratios;

[0014]FIG. 5A plots theoretical (002) interplanar spacing versus x-ray diffraction CrV/Cr (200) interplanar spacing;

[0015]FIG. 5B plots (002) interplanar spacing for CrV20/Cr chromium-based layers versus the calculated weighted CrV20/Cr (200) lattice spacing;

[0016]FIG. 6 plots CrMo20/Cr fraction and the calculated (002) spacing versus measured CrMo20/Cr (200) lattice spacing;

[0017]FIG. 7 plots calculated tCr/V50/(tCr+tCrV50) thickness fraction versus measured CrV50/Cr (002) interplanar spacing;

[0018]FIG. 8 plots the relationship between Mrt and thickness ratio of the thickness of CrV50 and of CrMo20 to the total underlayer thickness;

[0019]FIG. 8A plots the effect of t_(CrMo20)/t_(Cr) on LFTAA and PW50;

[0020]FIG. 8B plots the effect of t_(CrV50)/t_(Cr) on LFTAA and SMNR; and

[0021]FIG. 9 suggests the effect of different lattice parameters of two chromium-based layers on an underlayer.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0022]FIG. 3 is a simplified schematic illustration of a magnetic recording medium 16 made according to the invention. Magnetic recording medium 16 is similar disk 2 of FIG. 1 with like elements referred to by like reference numerals. However, underlayer 8A is made up of a first non-magnetic, chromium-based layer 18 and a second non-magnetic, chromium-based layer 20. Additional chromium-based layers can also be used. Not all of the chromium-based layers are the same. Thus, if chromium-based layer 18 is plain chromium, then chromium-based layer 20 is a chromium alloy, typically CrX (with a single alloying element) or CrXY (with two alloying elements). If desired, a chromium-based layer can include chromium and three or more alloying elements. The alloying elements may be a variety of elements but typically are chosen from the following group: C, Mg, Al, Si, Ti, V, Co, Ni, Cu, Zr, Mo, Nb, La, Ce, Nd, Gd, Tb, Dy, Er, Ta, W and Mn.

[0023] Each underlayer is typically a combination of two or more pure Cr, CrX or CrXY alloys as suggested in FIG. 3. The thicknesses of the chromium-based layers 18, 20 typically vary from about 1 nm to 50 nm. When deposited on a NiP substrate, the first deposited chromium-based layer 18 has a (002) fibrous texture and the subsequent chromium-based layer 20, and any additional chromium-based layers, assumes this (002) texture. When deposited on glass substrate, the first deposited Cr-based layer will assume the texture of the seedlayer. This texture is also copied by the subsequently deposited Cr-based layers. The lattice parameter of underlayer 8 can be varied through this multilayer construction. The experimental (002) interplanar spacing, as measured by x-ray diffraction, varies linearly with the theoretical calculated values; see FIGS. 5-7. This theoretical value is obtained by the weighted thickness and lattice parameter average of the two layers:

a _(underlayer) =a _(CrX1)(t _(CrX1) t _(Total))+a _(CrX2)(t _(CrX2) /t _(Total))  (1)

a: lattice parameter, t:film thickness. (t _(CrX1) +t _(CrX2) =t _(Total))

[0024] That is, the chromium-based layers 18, 20 have two different lattice parameters. Growing layer 20 on layer 18 causes the lattice parameter of the two layers to be pulled to an intermediate lattice parameter which becomes the lattice parameter of underlayer 8. This is shown graphically in FIG. 9 in which chromium-based layer 18 is a pure chromium layer having an a_(Cr) of 2.88 Å while layer 20 is a chromium vanadium alloy having an a_(CrV20) of 2.90Å. As indicated by the arrows, a smaller a _(Cr) of layer 18 has a tendency to expand while the larger a_(CrV20) of layer 20 is contracted so that the resulting lattice parameter of the underlayer will lie somewhere between the unrestrained lattice parameters of layers 18 and 20.

[0025] The change in the lattice parameter is measured by the peak shift in the position of the (002) peak in the x-ray diffraction spectra as illustrated in FIG. 4. The origin of this lattice dimension change apparently stems from the coherent strain, caused by the epitaxial growth between the two chromium-based layers. The resulting bcc unit cell dimension can be predicted through the thickness fraction, fCrX, of the CrX/Cr chromium-based layer (in this embodiment).

fCrX=tCrX/tCrX+tCr  (2)

[0026] By fine-tuning the lattice parameter of the underlayer using this multilayer construction, it is possible to improve the lattice matching between the magnetic layer and this combination underlayer. As a result, the in-plane magnetization ratio can be enhanced even though the magnetic and total underlayer thickness remains constant (FIG. 8).

I. Experimental Procedures

[0027] CoCrTaPt/CrX (X: V₂₀, V₅₀, Mo₁₀, Mo₂₀) films were sputtered onto NiP substrates in an Intevac MDP250 static sputtering system. The lattice parameter of the CrX underlayer films was measured to gauge the effect of alloying element additions and its relation to weighted average atomic radius.

[0028] Films with a dual underlayer design: CoCrTaPt/CrX/Cr (X: V₂₀, V₅₀, Mo₂₀) were also sputtered (FIG. 3). The CrX/Cr thickness ratio, were varied in order to induce the lattice dimension change in the layered underlayer. The total thickness of the underlayer and the CoCrTaPt layer were kept constant at 190±10 Å and 250 Å respectively. All the films were deposited at a substrate temperature of 260° C. The resulting films were characterized using θ-20 x-ray diffraction (XRD) and remnant moment magnetometer (RMM). Parametric performance was measured on a Guzik 1401 spinstand using MR heads.

II. Results and Discussion

[0029] A. CrX Alloys

[0030]FIG. 2 plots the relationship between the CrX (200) interplanar spacing (d₂₀₀) of the sputtered films as measured by XRD and theoretical CrX atomic radius (a_(CrX)). a_(CrX) was calculated using the weighted atomic radius average of Cr and element X:

a _(CrX) =a _(Cr)(1−r)+a _(x) r r: at. %  (3)

[0031] There is a perfect correspondence between a_(CrX) and experimental values which indicates that the lattice parameter of miscible Cr alloys can be predicted using a linear law of mixing of the Cr amd CrX atomic radius. The results can simplify composition selection of CrX alloys.

[0032] B. CrX/Cr Dual Underlayer

[0033]FIG. 4 shows the XRD spectra for different CrV₂₀/Cr underlayers with different t_(CrX)/t_(Cr). For pure Cr, the (002) peak would be located at 64.5. As the CrV₂₀ layer thickness is increased against a thinning Cr layer, the (002) peak shifts to a lower angle implying an expansion in the underlayer bcc lattice.

[0034] The measured d₂₀₀ is plotted against T_(CrV20)/T_(Cr) in FIG. 5B and it shows a linear relation for the most part. One can also calculate the thickness weighted average (002) interplanar spacing (a_(theo)) of the CrV₂₀/Cr underlayer: $\begin{matrix} {a_{theo} = \frac{{t_{Cr}*a_{Cr}} + {t_{CrX}*a_{CrX}}}{t_{total}}} & (4) \end{matrix}$

[0035] A good linear fit can be obtained between the experimental and calculated values (FIG. 5A) which indicates that not only can one change the CrV₂₀/Cr (002) underlayer lattice spacing through t_(CrV20)/t_(Cr) but one can also predict the resulting a once the upper and lower thickness ratio are defined.

[0036] The slope between measured results and that obtained from (4) is not unity in FIG. 5A. This can be attributed to the epitaxial strain between the two layers. CrV20 nanocrystalline grains grow epitaxially on Cr grains. The Cr grain size is replicated and both lattices are strained simultaneously. This consequently leads to the shift in the overall underlayer lattice parameter as shown above. Due to the nanocrystallinity of the grains and an abundance of strain relieving grain boundaries/free surfaces, the nature of the epitaxial strain is complex it cannot be easily predicted in a linear fashion. The may account for the shift between the measured and calculated values.

[0037] Similar linear trends were observed in CrMo₂₀/Cr and CrV₅₀/Cr dual underlayers between the measured spacing, thickness ratio and predicted values. However, the agreement between the calculated values and the measured spacing is not as good as the case with CrV₂₀/Cr. The relationship between the three quantities for CrMo₂₀/Cr and CrV₅₀/Cr are shown in FIGS. 6 and 7, respectively. Although it basically scales linearly with t_(CrMo20)/t_(Cr) and t_(CrV50)/t_(Cr), the range of the measured d₂₀₀ is much narrower than the one predicted using (4). This could be due to the larger lattice misfit between Cr and CrMo₂₀ and CrV₅₀ which makes the epitaxial strain more non-linear.

[0038]FIG. 8 plots the relationship between Mrt, fCrMo₂₀ and fCrV₅₀. A peak in Mrt can be observed in both curves. The experimental d₁₁₀ was found to be 2.092 Å and 2.107 Å respectively for CrMo₂₀/Cr and CrV₅₀/Cr. This falls within 0.8% of the (0002) spacing of CoCrTaPt, which is at 2.09Å. The good lattice matching gives a higher M_(r)t of almost 9% even though the magnetic film thickness remains constant. These results demonstrate that adjustment of the underlayer lattice using variable ratio layered underlayers is a viable alternative to optimizing the underlayer lattice dimension through binary alloy composition adjustment. The technique represents a less complex avenue to choosing the correct underlayer CrX composition.

[0039] The interplay between thickness ratio, low frequency track-average-amplitude (LFTAA), pulse width (PW₅₀), signal-to-media-noise-ratio (SMNR), fCrMo₂₀ and fCrV₅₀ is shown in FIGS. 8A and 8B. LFTAA is at or near maximum for the fraction which maximizes Mrt. However, PW₅₀ is not maximized nor is SMNR minimized, as one would expect according to normal parametric scaling. This is because the high amplitude arises from better matching between the magnetic layer and the underlayer but not from a thicker magnetic layer. This matching results in a narrower transition and better parametric performance.

III. Conclusions

[0040] The film lattice parameter, a, of CrX can be predicted using the weighted atomic radius between Cr and X, although this dimension can change depending on sputtering conditions. a can also be modified and adjusted through the thickness ratio in a CrX/Cr multilayer construction. The resulting a can be predicted using the weighted thickness average. Proper adjustment of this ratio leads to optimal magnetic and parametric properties.

[0041] Modification and variation can be made to the disclosed embodiments of the invention without departing from the subject of the invention as defined in the following claims. 

What is claimed is:
 1. A magnetic recording medium comprising: a substrate; an underlayer supported by the substrate, the underlaying comprising first and second non-magnetic, chromium-based layers, at least one of the first and second chromium-based layers being a chromium alloy, the compositions of the first and second chromium-based layers being different, the lattice parameter of the underlayer being between the lattice parameters of the first and second chromium-based layers; and a magnetic layer deposited on the underlayer.
 2. The magnetic recording medium according to claim 1 wherein the substrate is a chosen one of a NiP or a ceramic glass substrate, and the magnetic layer comprises a cobalt alloy.
 3. The magnetic recording medium according to claim 1 further comprising a protective layer deposited on the magnetic layer, a lubricant deposited on the protective layer and a non-magnetic seedlayer between the substrate and the underlayer.
 4. The magnetic recording medium according to claim 1 wherein the at least one of the chromium-based layers comprises an alloy chosen from the following group: C, Mg, Al, Si, Ti, V, Co, Ni, Cu, Zr, Mo, Nb, La, Ce, Nd, Gd, Tb, Dy, Er, Ta, W and Mn.
 5. The magnetic recording medium according to claim 1 wherein the thickness of the underlayer is about 180-200Å.
 6. The magnetic recording medium according to claim 1 wherein the thickness of the underlayer is less than about 350Å.
 7. The magnetic recording medium according to claim 1 wherein said first and second chromium-based layers are each between about 10 to 340Å.
 8. The magnetic recording medium according to claim 1 wherein the first-chromium-based layer is an all-chromium layer and the second chromium-based layer is a Cr/20% Mo alloy.
 9. A non-magnetic underlayer for a magnetic recording medium comprising: first and second non-magnetic, chromium-based layers, at least one of the first and second chromium-based layers being a chromium alloy, the compositions of the first and second chromium-based layers being different, the lattice parameter of the underlayer being between the lattice parameters of the first and second chromium-based layers.
 10. A method for obtaining a desired lattice parameter for a non-magnetic underlayer in the manufacture of a magnetic recording medium of the type comprising a substrate, a non-magnetic underlayer on the substrate and a magnetic layer on the underlayer, comprising: determining a desired lattice parameter for the underlayer, the underlayer having a thickness; selecting a plurality of non-magnetic, chromium-based layers for the underlayer, at least one of said chromium-based layers being a chromium alloy; and choosing a fractional thickness for each of the chromium-based layers according to the desired lattice parameter, the sum of the fractional thicknesses for each of the chromium-based layers being equal to the underlayer thickness.
 11. The magnetic recording medium according to claim 10 wherein the selecting step selects two said chromium-based layers.
 12. The magnetic recording medium according to claim 11 wherein the choosing step is carried out using a ratio of the two fractional thickness of said two chromium-based layers versus corresponding (002) interplanar spacing as the lattice parameter.
 13. The magnetic recording medium according to claim 12 wherein the choosing step is carried out using a plot of said ratio versus (002) interplanar spacing.
 14. The magnetic recording medium according to claim 10 wherein the choosing step is carried out so that the difference between the lattice parameter of the underlayer and the desired lattice parameter is optimized.
 15. The magnetic recording medium according to claim 11 wherein the selecting and choosing steps are carried out so the lattice parameter of the underlayer is between the lattice parameters of the chromium-based layers.
 16. The magnetic recording medium according to claim 11 wherein the determining step is carried out with the desired lattice parameter being the same as the lattice parameter of the magnetic layer.
 17. A method for obtaining a desired lattice parameter of a non-magnetic underlayer in the manufacture of a magnetic recording medium of the type comprising a substrate, a non-magnetic underlayer on the substrate and a magnetic layer on the underlayer, comprising: determining a desired lattice parameter for the underlayer, the underlayer having a thickness, the desired lattice parameter being the same as the lattice parameter of the magnetic layer; selecting first and second non-magnetic, chromium-based layers for the underlayer, at least one of said chromium-based layers being a chromium-based alloy; and choosing a fractional thickness for each of the chromium-based layers according to the desired lattice parameter using a ratio of the fractional thicknesses for said layers versus the lattice parameter, the sum of the fractional thicknesses for each of the chromium-based layers being equal to the underlayer thickness. 